A laser printer generates an image on a piece of paper by scanning a focused laser beam over a cylindrical photosensitive drum, the signal directed to the laser beam for each pixel controlling the length of time which the laser beam is on. The drum converts the laser power incident on the drum to electrostatic charges, and the electrostatic charges attract and retain a powdered ink or "toner" (generically referred to as a "marking medium"). When an electrostatically charged paper (generically referred to as a "printing surface") is rolled against the drum, the toner is transferred to the paper, and the paper is then heated to fuse the toner to the paper. The resolution of the image is determined by the number of scan lines per inch (pi) and the density of pixel values along each scan line, as measured in dots per inch (dpi). A typical high-resolution laser printer has 600 scan lines per inch and 600 dots per inch along each scan line.
A graph of an exemplary laser signal 100 as a function of pixel position is shown in FIG. 1A, and a graph of the resulting line thickness 130 and a magnified view of the toner line 170 generated by the laser printer are shown in FIGS. 1B and 1C, respectively. (For clarity, the toner line 170 is shown with no toner in the scan lines 160 and 180 directly above and below it 170, respectively, although generally there will be toner in those scan lines 160 and 180 as well.) As may be noted by inspection of FIGS. 1A-C, the thickness of the toner line 170 is roughly proportional to the signal level directed to the laser. The thickness of the toner line 170 reaches a width of one pixel when the level of the laser signal has its full-pixel-width value, PP, and the thickness of the line 170 having a maximum toner width, WM, (which is three pixels in the example of FIG. 1C) when the level of the laser signal has its maximum value, PM.
While a monochromatic image will only require one toner line per scan line, a full-color image will require multiple colored inks per scan line. Most commonly, fill-color printed images are generated using cyan, magenta, yellow and black inks, and the image is referred to as the "CMYK" image. Although the present invention applies to full-color image printing as well as monochromatic image printing, for ease of discussion of the present invention only one toner color per scan line is discussed, since the generalization to multiple toners per scan line is straightforward.
Although laser printers can generate images quickly, nonlinearities in the relation between the level directed to the laser and the amount of deposited toner are problematic in the accurate generation of images, especially in regions where the toner density is low (highlight regions) and regions where the toner density is high (shadow regions). One of the nonlinearities associated with the process is the result of interparticulate attraction between toner particles which causes them to clump together, preventing dots of toner deposited on the paper from being below a minimum size. Another cause of nonlinearities between the signal applied to the laser and the resulting toner density is a result of the time required to turn the laser off and on. As illustrated by the graph of the laser signal as a function of pixel number of FIG. 1D and the corresponding graph of incident laser power as a function of distance on the toner drum of FIG. 1E, the length of time that the laser power signal has the maximum laser power value (which is normalized to a value of unity in FIG. 1E) is roughly proportional to the laser signal value. However, the laser power cannot reach the value of unity instantaneously, nor cannot it return to zero power instantaneously. These ramp times effect the proportionality between the laser signal and the laser power. The non-proportionality is most dramatic for small laser signals, such as the signal at pixel number 1, where the finite-length rise and fall times of the laser power prevent the laser power from reaching the unity value.
A graph of an exemplary low-power laser signal 200 is shown in FIG. 2A, and a graph of the resulting line thickness 230 and a magnified view of the toner line 260 generated by the laser printer are shown in FIGS. 2B and 2C, respectively. As may be noted by inspection of FIGS. 2A-C, for pixels which are surrounded by laser values of zero and have laser values which are below an isolated-single-pixel cutoff value, PC1, such as the second pixel, the laser engine cannot deposit any toner on the paper. However, for a pixel which is surrounded by laser values of zero and is above the isolated-single-pixel cutoff value PC1 (such as the fourth pixel), the laser engine can generate a toner covered area 262. Similarly, the laser engine cannot generate a toner covered area for a pair of laser values which are surrounded by laser values of zero, and for which the average value is below an isolated-double-pixels cutoff value, PC2. However, when an isolated pair of pixels, such as the sixth and seventh pixels, have an average value above the isolated-double-pixels cutoff value PC2, a toner covered area 264 is produced. Similarly, the isolated-triple-pixels cutoff PC3 has a value below the isolated-double-pixels cutoff value PC2, and when an isolated triple of pixels (not shown) have an average value above the isolated-triple-pixels cutoff value PC3, a toner covered area is produced, and so on.
The situation is actually somewhat more complex than indicated above, since non-zero values of surrounding pixels influence the cutoff values. For instance, when the laser values surrounding a pixel have a value of one, the smallest printable dot corresponds to a value somewhat less than PC1. And when the laser values surrounding the dot have a value of two, the smallest printable dot corresponds to a value somewhat less still. Similarly, when the laser values surrounding a pair of pixels have a value of one, the smallest printable dot corresponds to an average value of the pair of pixels somewhat less than PC2.
Another nonlinearity of laser printer imaging is found in the larger laser values which produce shadowed regions. A graph of an exemplary laser signal 300 is shown in FIG. 3A, and a graph of the resulting line thickness 330 and a magnified view of the toner line 360 generated by the laser printer are shown in FIGS. 3B and 3C, respectively. As may be noted by inspection of FIGS. 3A-C, for laser values 300 which are near the middle of the range between zero and the full-pixel-width value PP (in this case the first through sixth pixels), the laser engine generates a line 360 whose thickness is roughly proportional to the laser values. However, for laser values which are near the full-pixel-width value PP (such as those of the seventh, eighth and ninth pixels), the laser engine tends to overly darken the pixels.
The underemphasis of low-power laser signals and the overemphasis of high-power laser signals (in scan lines of uniform darkness) is represented by the graph 400 of FIG. 4A, which plots laser value versus toner line thickness, i.e., toner "density." As can be seen from the graph 400, for low values 410 of the laser signal the line thickness is zero, as discussed above in reference to FIGS. 2A-C, and for high values 430 of the laser signal the line thickness reaches unity prior to the laser signal reaching the value PP, as discussed above in reference to FIGS. 3A-C.
Another drawback illustrated by FIG. 4A, is the fact that the laser values have a finite number of discrete values and the line thicknesses therefore have a finite number of discrete thicknesses. This "quantization" of line thicknesses can produce "false contours" when there is an area over which the color changes slowly--false contours occur at boundaries between a region having a gray scale of a first level and an adjoining region having a gray scale at an adjacent level.
These non-linearities and quantization effects makes accurate image printing a difficult and complex problem. An object of the present invention is therefore to provide method, apparatus, system and media for laser printing which has accurate tonal reproduction and is visually pleasing.
More particularly, it is an object of the present invention to provide method, apparatus, system and media for accurate laser printing in areas with low toner densities.
It is another object of the present invention to provide method, apparatus, system and media for accurate laser printing in areas with high toner densities.
It is another object of the present invention is to provide method, apparatus, system and media for laser printing with smooth transitions between gray scales, especially in areas where the image gray scale changes slowly.